Electric-wave absorber composition

ABSTRACT

There is disclosed a composition which is layered on a reflective board to constitute a radio wave absorber. In this composition, an electroconductive titanium oxide is compounded with a substrate. With regard to the substrate, there may be used thermoplastic resin, thermosetting resin, rubber or elastomer.

TECHNICAL FIELD

[0001] This invention relates to a radio wave absorber which is widelyused for shipping, airplanes, etc. and, more particularly, it relates toa composition of a radio wave absorber of a single layer type which isapplied or placed on a reflector made of metal or the like.

BACKGROUND OF THE INVENTION

[0002] In recent years, studies for utilization of radio wave of amicrowave range and a millimetric wave range have been briskly carriedout in various fields and, as a result, radio wave absorbers forprevention of electromagnetic interference have been receiving publicattention.

[0003] Among them, wavelength of radio wave of a millimetric wave rangeis particularly as very short as about 1-10 mm and, therefore, in theso-called “absorption of matching type” where radio wave is absorbed byan absorber placed on the surface of a reflector, there has been arequirement that thickness of the radio wave absorber matching suchradio wave is to be made 1 mm or less.

[0004] Further, there has been a dilemma in the radio wave absorbercompositions up to now that the radio wave absorber having a thicknessof some extent is necessary when the radio wave absorbing property for abroad range of radio wave is to be achieved while, when thickness of theradio wave absorber is to be made thin, radio wave absorbing propertyfor a broad range is not available.

DISCLOSURE OF THE INVENTION

[0005] An object of this invention is to provide a composition of aradio wave absorber which is able to efficiently absorb the radio waveof a broad range in spite of the fact that it is a radio wave absorberhaving a thin thickness.

[0006] A gist of the radio wave absorber composition of this inventionis that an electroconductive titanium oxide is compounded with asubstrate for achieving the above-mentioned object.

[0007] It is preferred that the compounding ratio of the above-mentionedelectroconductive titanium oxide to 100 parts by weight of the substrateis 5-40 parts by weight and it is also preferred that anelectroconductive carbon black is compounded in an amount of from morethan 0 part by weight to not more than 4 parts by weight to 100 parts byweight of the substrate.

[0008] The radio wave absorber composition of this invention isconstituted as mentioned above and, when it is applied or placed on areflector made of metal or the like, it is possible that a radio waveabsorber corresponding to any frequency of a microwave range and amillimetric wave range is prepared depending upon the changes in itsthickness and it is also possible that radio wave of a broad range canbe efficiently absorbed in spite of the radio wave absorber having athin thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a graph showing the result of measurement of complexrelative permittivity of a sample of an epoxy resin board in accordancewith this invention.

[0010]FIG. 2 is another graph showing the result of measurement ofcomplex relative permittivity of a sample of an epoxy resin board inaccordance with this invention.

[0011]FIG. 3 is a graph showing the result of evaluation of radioabsorbing property of a coating material according to an example of thisinvention.

[0012]FIG. 4 is a graph showing the result of evaluation of radioabsorbing property of a coating material according to another example ofthis invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0013] As hereunder, embodiments of this invention will be illustratedin detail by referring to the drawings.

[0014] The radio wave absorber composition of this invention is preparedby compounding an electroconductive titanium oxide and, if desired, anelectroconductive carbon black with a substrate.

[0015] With regard to the substrate, a material which is selected from,for example, thermoplastic resin, thermosetting resin, various kinds ofrubbers and elastomer is used. Particularly, a thermosetting resin isable to easily constitute a radio wave absorber having a desiredthickness by applying on a reflector followed by setting even when thereflector for a radio wave absorber has projections.

[0016] With regard to the above-mentioned thermoplastic resin, it ispossible to use, for example, polyolefin resin such as polyethylene andpolypropylene; polyamide resin such as Nylon 6 and Nylon 66; polyesterresin such as polyethylene terephthalate and polybutylene terephthalate;or a mixture thereof.

[0017] With regard to the thermosetting resin, it is appropriatelyselected from, for example, epoxy resin, polyurethane resin, polyesterresin and phenol resin. With regard to a setting agent, that which iscommonly used is used and, therefore, there is no particular limitationfor its type and quantity.

[0018] The above-mentioned electroconductive titanium oxide is thatwhere the surface of titanium oxide comprising spherical crystals of arutile type or needle crystals of a rutile type is coated with anelectroconductive layer such as SnO₂/Sb layer. This electroconductivetitanium oxide is dispersed in a substrate to make the permittivity ofthe said substrate high. In addition, the above-mentionedelectroconductive titanium oxide has an excellent physical and chemicalstability and also has an excellent dispersibility when compounded witha substrate.

[0019] With regard to the above-mentioned carbon black, there may beexemplified acetylene black, furnace black and Ketjenblack. Ketjenblackis particularly preferred. The electroconductive carbon black isdispersed in a substrate so as to make the permittivity of the saidsubstrate high.

[0020] The compounding amount of the electroconductive titanium oxideand the electroconductive carbon black for achieving the most idealradio wave absorbing property is determined by frequency of the radiowave and also by complex relative permittivity of the absorbercorresponding thereto.

[0021] That will be illustrated on the basis of an experiment. Thus,epoxy resin and a setting agent were used as a substrate, then anelectroconductive titanium oxide and an electroconductive carbon blackin six types of weight ratios to 100 parts by weight of the substrate asshown in Table 1 were added thereto as fillers followed by stirring, themixture was flown into a container to prepare a resin board samples(samples 1-6) of 150 mm ×150 mm ×2 mm (thickness) and their complexrelative permittivities at 50-110 GHz (angle of incidence: 0°) weremeasured by a measuring apparatus for complex relative permittivity infree space manufactured by VHS. In this apparatus, radio wave whichtransmits the sample is subjected to an impedance measurement by avector network analyzer whereby a complex relative permittivity iscalculated. The result is shown in FIG.1.

[0022] The curve shown by a dotted line in FIG. 1 is a non-reflectivecurve expressing a complex relative permittivity requested for amaterial which is to be non-reflective and it shows that, when amaterial where a complex relative permittivity is preset on thenon-reflective curve is used, the most ideal radio wave absorber can beobtained.

[0023] It is also possible that, on the non-reflective curve, ratio(d/λ) of the wavelength λ (mm) to the sample thickness d (mm) is plottedand the value of d/λ is determined by the complex relative permittivityof the sample. Accordingly, it is possible to determine the thickness ofthe radio wave absorber depending upon the wavelength of the objectivewavelength. TABLE 1 Electroconductive Electroconductive Sample TitaniumOxide Carbon Black 1 20 0 2 20 1 3 20 2 4 30 1 5 32 1 6 10 0

[0024] From the result of FIG. 1, the following facts have been found.

[0025] (1) When the samples 1, 2 and 3 are compared, both real andimaginary parts of the complex relative permittivity increase when theadding amount of the electroconductive carbon black is increased and theratio is bigger in the real part. Accordingly, when a small amount ofthe electroconductive carbon black is added, it comes nearer the sidewhere (d/λ) on the non-reflective curve becomes smaller whereby athinner absorber can be prepared. It has been known that, when theadding amount of the electroconductive carbon black is furtherincreased, there is an increase of the ratio where the imaginary partincreases.

[0026] (2) In the samples 5 and 6, the complex relative permittivity ison a non-reflective curve and that is a compounding having an idealabsorbing property. Especially in the sample 5, the ratio (d/λ) ofthickness of the absorber to wavelength of the absorbed wavelength isabout 0.09 whereby a thin absorber can be prepared.

[0027] (3) In each of the samples, the complex relative permittivitychanges nearly in parallel to the non-reflective curve together withfrequency and, therefore, it is possible to prepare an absorbercorresponding to any frequency of 50-110 GHz by a mere change ofthickness without changing the compounding.

[0028] From the above results, the sample 5 which is most suitable forthe object of this invention was selected and a complex relativepermittivity at 5-50 GHz was measured as same as above by expanding thefrequency range to a microwave range and the result is shown in FIG. 2together with the above-mentioned results.

[0029] As a result, it was confirmed that the complex relativepermittivity of the sample 5 was almost close to the non-reflectivecurve even at the frequency range of 5-50 GHz and that the compositionof the sample 5 efficiently absorbed the radio wave of microwave andmillimetric wave ranges.

[0030] From the above, in a composition of the radio wave absorber, itis preferred that the compounding amount of the electroconductivetitanium oxide to 100 parts by weight of the substrate is 5-40 parts byweight or, preferably, 10-35 parts by weight and that the compoundingamount of the electroconductive carbon black to 100 parts by weight ofthe substrate is from more than 0 part by weight to not more than 4parts by weight while the variation factors by the type of the substrateand also by the type of the electroconductive titanium oxide and theelectroconductive carbon black are taken into consideration. Further,thickness of the radio wave absorber is determined by the relationbetween the complex relative permittivity and the wavelength of theradio wave to be absorbed.

[0031] Here, the electroconductive titanium oxide is effective formaking the range of the absorbing property broad but, when itscompounding amount to 100 parts by weight of the substrate is less than5 parts by weight, both real and imaginary parts of the complex relativepermittivity of the material become too low whereby the material is notable to cope with the radio wave of microwave range and millimetric waverange while, when the said amount is more than 40 parts by weight, bothreal and imaginary parts of the complex relative permittivity become toohigh whereby the material is not able to cope with those radio waves.

[0032] In addition, compounding of the electroconductive carbon black ispreferred since it makes both real and imaginary parts of the complexrelative permittivity high whereby thickness of the radio wave absorbercan be made thin without affecting the radio wave absorbing property.However, when its compounding amount to 100 parts by weight of thesubstrate is more than 4 parts by weight, viscosity of the materialbecomes high whereby the processing ability becomes bad. In addition, anincreasing rate of the imaginary part of the complex relativepermittivity increases and, therefore, the complex relative permittivitytakes a value apart from the non-reflective curve and the material doesnot cope with the radio wave.

[0033] When the coating material obtained hereinabove is applied on themetal part of shipping followed by setting or a film comprising theabove-mentioned composition is placed on metal parts of the shipping,radio wave of microwave range or millimetric wave range can beefficiently absorbed.

[0034] On the other hand, there are many cases in some materials to becoated that weight of the radio wave absorber causes an obstacle wherebythere are some cases where it is demanded to make the weight of theradio wave absorber light. In that case, a foamed substance is containedin the composition of the radio wave absorber.

[0035] When a foamed substance is contained, an air phase increases inthe inner area of the radio wave absorber and the permittivity lowers.Therefore, it is necessary to consider in previously increasing theadding amount of the electroconductive titanium oxide to make thepermittivity high.

[0036] With regard to the foamed substance, Microballoon is preferablyused. The amount of Microballoon to 100 parts by weight of the substrateis from more than 0 part by weight to not more than 30 parts by weightor, preferably, not more than 20 parts by weight. When the amount ismore than 30 parts by weight, permittivity of the material lowerswhereby a desired radio wave absorbing property is not achieved.

EXAMPLES Example 1

[0037] A mixture of epoxy resin and a setting agent was used as asubstrate, then an electroconductive titanium oxide and anelectroconductive carbon black were added in the amounts as shown inTable 2, then a solvent was added thereto and the mixture was mixed withstirring in a mill. Amount of the solvent to 80 parts by weight of theabove composition was 20 parts by weight.

[0038] This was applied by spraying on an aluminum board until thethickness after setting became 3.60 mm (sample 7), 0.75 mm (sample 8),0.40 mm (sample 9), 0.35 mm (sample 10) and 0.31 mm (sample 11)whereupon five sheets of flat boards (samples 7-11) were prepared. Afterthe coating material was set, radio wave of microwave range andmillimetric wave range (frequency: 1-110 GHz) was subjected to incidenceonto the applied surface of the flat board with an angle of incidence of4° whereupon the absorbing property was evaluated. The result is shownin FIG. 3. TABLE 2 Parts by Weight Substrate (Epoxy Resin and SettingAgent) 100 Electroconductive Titanium Oxide (*1) 32 ElectroconductiveCarbon Black (*2) 1

[0039] From FIG. 3, it was found that, in each of the samples, maximumvalue of the absorbing property was not less than 25 dB and,particularly in the samples 9, 10 and 11, the frequency range showing anabsorbing property of not less than 20 dB which was a yardstick for anexcellent absorbing property was confirmed within a broad range offrequency region and that the samples were good radio wave absorbers.

Example 2

[0040] A mixture of epoxy resin and a setting agent was used as asubstrate, then an electroconductive titanium oxide, anelectroconductive carbon black and Microballoon were added thereto inthe amounts as shown in Table 3 followed by adding a solvent thereto andthe mixture was mixed with stirring in a mill. Amount of the solvent to80 parts by weight of the above composition was 20 parts by weight.

[0041] This was applied by spraying on an aluminum board until thethickness after setting became 2.4 mm. After the coating material wasset, radio wave of microwave range (8-12 GHz) was subjected to incidenceonto the applied surface with an angle of incidence of 4° whereupon theabsorbing property was evaluated. The result is shown in FIG. 4. TABLE 3Parts by Weight Substrate (Epoxy Resin and Setting Agent) 100Electroconductive Titanium Oxide (*1) 40 Electroconductive Carbon Black(*2) 1 Microballoon (*3) 14.3

[0042] From FIG. 4, it was found that maximum value of the absorbingproperty was not less than 25 dB and the frequency range showing anabsorbing property of not less than 20 dB which was a yardstick for anexcellent absorbing property was confirmed within a certain extent.

Industrial Applicability

[0043] In this invention, a composition where an electroconductivetitanium oxide is compounded with a substrate such as resin is used as amaterial for absorption of radio wave as mentioned above whereupon it isnow possible that a radio wave absorber corresponding to any frequencyof a microwave range and a millimetric wave range is prepared dependingupon the changes in its thickness and it is also possible that radiowave of a broad range can be efficiently absorbed in spite of the radiowave absorber having a thin thickness.

[0044] In addition, when Microballoon is compounded with theabove-mentioned substrate, it is possible to make the weight of theradio wave absorber light.

What is claimed is:
 1. A composition for absorption of radio wave wherean electroconductive titanium oxide is compounded with a substrate. 2.The composition for absorption of radio wave according to claim 1,wherein the compounding rate of the said electroconductive titaniumoxide to 100 parts by weight of the substrate is 5-40 parts by weight.3. The composition for absorption of radio wave according to claim 1,wherein an electroconductive carbon black is compounded in an amount offrom more than 0 part by weight to not more than 4 parts by weight to100 parts by weight of the substrate.
 4. The composition for absorptionof radio wave according to claim 2, wherein an electroconductive carbonblack is compounded in an amount of from more than 0 part by weight tonot more than 4 parts by weight to 100 parts by weight of the substrate.5. The composition for absorption of radio wave according to any ofclaims 1 to 4, wherein the said substrate is at least one memberselected from a group consisting of thermoplastic resin, thermosettingresin, rubber and elastomer.
 6. The composition for absorption of radiowave according to any of claims 1 to 4, wherein the said substrate is athermosetting resin.